Sap- suck­ers on the prowl

Sunday Tasmanian - Tassie Living - - HOME - Peter Cun­dall

PROB­A­BLY the most wor­ry­ing of all plants pests right now are scale in­sects. There’s a huge range of them and they come in all shapes, sizes and gen­eral ap­pear­ances. Th­ese fast- breed­ing sap- suck­ers cause a great deal of dis­tress be­cause of the dam­age they can do and in some cases the mess they cre­ate.

For ex­am­ple, cit­rus trees are highly vul­ner­a­ble to at­tacks from soft scale. We can see th­ese as masses of tiny lumps along the youngest shoots and clus­tered along the midribs of leaves – mainly un­der­neath and out of sight.

They feed con­stantly on the sap and se­ri­ous in­fes­ta­tions will weaken trees and stunt growth. They pro­duce honey- dew which also at­tracts ants to feed off it. This sticky sub­stance also falls on leaves, then as it ages be­comes an un­sightly, black sooty mould that cov­ers leaves and fruit.

Of­ten this is the first in­di­ca­tion that a tree has be­come in­fested with soft scale and many in­ex­pe­ri­enced gar­den­ers mis­tak­enly be­lieve that the black- en­crusted fo­liage is the ma­jor prob­lem.

A safe means of con­trol is to spray the en­tire canopy – in­clud­ing be­neath all leaves – with di­luted white oil emul­sion or pest oil. Two thor­ough sprays at five- day in­ter­vals are enough to suf­fo­cate the pests, al­though the sooty mould will per­sist un­til grad­u­ally washed off by rain or ir­ri­ga­tion wa­ter.

Other scales have a com­pletely dif­fer­ent ap­pear­ance. Cot­tony cush­ion scale is now at­tack­ing many ap­ple trees, es­pe­cially the youngest shoots and leaves. They look like small, well- sep­a­rated blobs of cot­ton wool, cling­ing to the un­der­sides of leaves. Good con­trol can be achieved by prun­ing out all in­fested leaves and branches, which would be nor­mal sum­mer prun­ing any­way. Be sure to rake up and cart away all de­bris af­ter­wards.

Cot­tony cush­ion scale closely re­sem­bles another pest known as hy­drangea scale. Old, un­pruned and ne­glected hy­drangeas, es­pe­cially those un­der some kind of stress due to lack of wa­ter, are usu­ally the main tar­gets.

The white, cot­tony scale bod­ies are clearly vis­i­ble on all parts of hy­drangea bushes – usu­ally mop- tops – in­clud­ing older stems, new shoots and leaves. In fact, a badly in­fested hy­drangea will be dot­ted with hun­dreds of th­ese un­sightly white blobs.

The ob­vi­ous treat­ment to con­trol hy­drangea scale is to first wa­ter plants so roots are heav­ily soaked. Then prune out all old, woody branches al­most down to the ground. Fi­nally, spray with di­luted white oil emul­sion or pest oil to kill any re­main­ing pests.

This sea­son has seen an in­crease in a com­mon ap­ple tree pest that looks a bit like the cot­tony cush­ion scale. In fact it is a species of aphid that cov­ers it­self with masses of fine, waxy threads that look like white wool – and is in fact called woolly aphid.

When clus­tered to­gether, woolly aphids look as though some­one has stuck masses of cot­ton wool along the un­der­sides of branches.

This pest tends to con­cen­trate in ar­eas where branches have been wounded by wind or prun­ing. As they suck the sap, the trees con­stantly try to re­pair the dam­age by pro­duc­ing lumpy, dis­tort­ing cal­luses along in­fested branches.

A quick and easy means of con­trol is to use an old paint­brush dipped in methy­lated spirit to paint the feed­ing clus­ters. This in­stantly kills the pests, al­though this can be a te­dious job with a large, badly in­fested tree.

Many va­ri­eties of crabap­ple trees are highly sus­cep­ti­ble to woolly aphid at­tack. Take par­tic­u­lar care to paint the area be­low graft unions on all ap­ple va­ri­eties be­cause this stock wood is a com­mon breed­ing and over-win­ter­ing place for th­ese pests.

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